This invention pertains to microlithography (pattern projection and transfer from a mask or reticle to a substrate) as used in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to microlithography using a charged particle beam (e.g., electron beam or ion beam) to transfer a circuit pattern or the like to a substrate (e.g., semiconductor wafer) at a resolution (minimum linewidth) of 0.1 xcexcm or less on the substrate.
As feature sizes in circuit patterns for integrated circuits, displays, and the like progressively have been miniaturized, the resolution limits of optical microlithography have become increasingly apparent. This has resulted in intensive efforts to develop practical microlithography apparatus and methods exploiting an exposure technology offering prospects of substantially greater resolution than obtainable using optical microlithography. Optical microlithography utilizes a beam of light (typically ultraviolet light) as a pattern-transfer energy beam. One candidate alternative technology to optical microlithography involves the use of a charged particle beam (e.g., electron beam or ion beam) rather than a light beam as an energy beam.
Whereas charged-particle-beam (CPB) microlithography (e.g., electron-beam microlithography) offers prospects of high resolution, many technical problems must be solved in order to develop practical CPB microlithography apparatus and methods. One technical problem pertains to beam drift, i.e., changes in actual beam position relative to desired beam position. As can be surmised, in order to achieve a pattern-feature resolution on the order of 0.1 xcexcm or less, the position of a charged particle beam as used for pattern transfer must be controlled extremely accurately and precisely. If beam drift is excessive, then the xe2x80x9cCPB optical systemxe2x80x9d (i.e., assembly of xe2x80x9clensesxe2x80x9d, deflectors, and the like for shaping and guiding the beam from a source to the substrate) conventionally must be disassembled, cleaned, and reassembled.
Most instances of beam drift arise from the accumulation of contaminants in the CPB optical system. Deposits of contaminants in a CPB optical system tend to accumulate static charges that can have a significant electrostatic effect on the beam. I.e., propagation of the beam past contaminant deposits presenting an unwanted electrostatic charge to the beam can cause the beam to be deflected or distorted in undesirable ways. Some causes of beam drift can be attributed to parameters that can be controlled in the optical system such as variations in lens-induction current, deflection current, voltage, temperature, and the like. Nevertheless, beam drift (especially beam drift caused by factors that cannot be controlled directly) remains an important problem requiring effective solution.
Whereas the beam current in certain types of CPB microlithography apparatus (specifically, conventional electron-beam xe2x80x9cvariable-shaped beam apparatusxe2x80x9d) is usually small (approximately 1 xcexcA or less), a beam current of, e.g., 20 times greater (i.e., 20 to 25 xcexcA) is used in other types of apparatus such as xe2x80x9cdivided-patternxe2x80x9d CPB microlithography. Exposure of a resist on the surface of a wafer with these higher beam currents typically generates large amounts of volatile by-products of the resist. The volatile by-products tend to deposit in various locations inside the CPB optical system, and the rate of deposition tends to increase with increases in beam current. To achieve and maintain maximal resolution of pattern transfer, beam stability (freedom from significant drift) must be maintained at a high level. However, to maintain such stability at high beam currents, affected components of the CPB optical system must be disassembled and cleaned progressively more frequently. In addition, especially at higher beam currents, resulting variations in temperature of the components in the CPB optical system can cause significant beam drift, even in instances in which the CPB optical system is not xe2x80x9cdirty.xe2x80x9d
This invention addresses these problems and its purpose is to provide, inter alia, methods for realizing high-precision pattern transfer, even when there is a certain amount of beam drift.
A charged-particle-beam CPB) microlithography (xe2x80x9cprojection-transferxe2x80x9d or xe2x80x9cprojection-exposurexe2x80x9d) system according to the invention employs a reticle in which the pattern field is divided or xe2x80x9csegmentedxe2x80x9d into multiple portions defining respective portions of the pattern. More specifically, the pattern field is divided into multiple xe2x80x9cstripesxe2x80x9d that are typically rectangular in shape. Each stripe has a width (shorter dimension) that is within the deflectable field of the CPB optical system. Each stripe is divided further into multiple parallel xe2x80x9cdeflection fieldsxe2x80x9d each having a length extending the width of the stripe. The overall pattern field is transferred stripe-by-stripe and each stripe is transferred deflection-strip-by-deflection-strip. To transfer a stripe, the charged particle beam is deflected, in a scanning manner, across the width of the stripe to scanningly expose each deflection field. As each deflection field is exposed, the reticle and substrate are mechanically displaced as required (in the length dimension of the stripe) to bring the next deflection field into position for exposure.
The reticle includes beam-drift test patterns in certain deflection fields (a beam-drift test pattern desirably is located in a terminus of the respective deflection field) that is scanned by the charged particle beam (functioning as a xe2x80x9cdetection beamxe2x80x9d). Corresponding beam-test marks are disposed on or at a substrate in locations where the respective test-pattern-containing deflection fields will be exposed by the charged particle beam. The beam-test marks on the substrate are irradiated scanningly by the detection beam (passing through corresponding beam-test patterns on the reticle) prior to pattern transfer. The positions of the test patterns are detected (desirably iteratively) by scanning the corresponding beam-test marks on the substrate multiple times with the detection beam passing through the corresponding beam-drift test patterns on the reticle, thereby providing positional data for detecting beam drift. From such positional data, the magnitude and directions of corrective deflections to the beam are calculated. The beam position during subsequent pattern transfer is corrected according to the results of these calculations to correct the beam drift and achieve a more accurate pattern transfer.
Whenever the charged particle beam is being used constantly under identical conditions, the magnitude of beam drift over time tends to be minimal. On the other hand, comparatively large beam drifts tend to occur whenever beam parameters are changed. For example, substantial beam drift can occur immediately after resuming use of a beam that has been xe2x80x9cblankedxe2x80x9d for a long period of time. Substantial beam drift also can occur whenever the beam current is suddenly and substantially changed from a high beam current to a low beam current, for example, or immediately after the beam is subjected to a large deflection. Even though such drifts are regarded as irregular, a degree of repeatability can be discerned in them under similar beam parameters.
In CPB microlithography apparatus that perform pattern transfer using reticles having identical specifications, essentially the same exposure operations normally are repeated at locations on the reticle at which the beam is either blanked or deflected, but not at locations on the reticle at which the beam current normally changes with a change in the pattern. Similar magnitudes and directions of the beam drift tend to be evident at the respective repeated locations. By correcting these repeatable components of beam drift, it is possible to perform high-accuracy pattern transfer even when there is a small residual amount of beam drift.
According to the invention, changing ratios of beam drift or simple differences in beam drift observed in various deflection fields can be measured and tabulated in advance using a test reticle defining a beam-drift test pattern in multiple deflection fields. Also, a test substrate can be used that possesses corresponding beam-test marks in respective regions on the substrate corresponding to the deflection fields on the reticle. Actual pattern transfer can be performed after storing these data (after replacing the test reticle and test substrate with an actual transfer reticle and substrate). During such exposure, beam drift is corrected based on the pre-tabulated ratios or differences.
Alternatively to using a test reticle and test substrate, it is possible to dispose beam-drift test patterns on a region of a normal patterned reticle and corresponding beam-test marks on a corresponding region of an actual wafer. In such a situation, beam drift can be measured in real time during actual pattern transfer. In any event, with a CPB microlithography apparatus used full time for semiconductor device fabrication, it is desirable to measure beam drift on a regular basis, such as at least once a week, using a dedicated test reticle and test wafer. The results of such periodic tests desirably are tabulated and used to perform corrections, as required, of beam drift during the remaining time the apparatus is used. By using a dedicated test reticle, more of an actual production reticle can be used for defining the pattern to be transferred to wafers for actual device manufacture.
It is also desirable for the feature density in the deflection fields containing the beam-drift test pattern or in the corresponding beam-test marks on the substrate to be substantially equal to the average feature density of an actual device pattern to be transferred.
Whenever beam current is high, the magnitude of beam drift tends to be correspondingly larger, and the opposite is experienced whenever the beam current is low. Nevertheless, beam drift can be measured under test conditions that are nearly identical to conditions during actual device-pattern transfer, and beam-drift correction under such conditions can be performed very accurately. Beam drift also can be measured while changing the beam current to various levels of magnitude, and the beam-drift correction can be calculated as a function of the beam current. Since beam current and drift magnitude may not be proportional, drift magnitude alternatively can be determined by interpolating from a mid-range value of beam current.
A CPB microlithography (pattern-transfer exposure) apparatus according to the invention comprises an illumination-optical system situated and configured to illuminate a reticle defining features of a pattern to be transferred to a sensitive substrate. The reticle is mounted on a movable reticle stage and illuminated with an xe2x80x9cillumination beam.xe2x80x9d The apparatus also comprises a projection-optical system situated and configured to project and form an image of the portion of the illumination beam (that has passed through the illuminated portion of the reticle and become a xe2x80x9cpatterned beamxe2x80x9d) onto a desired location on the sensitive substrate. The substrate is mounted on a movable substrate stage.
A reticle according to the invention comprises a pattern-transfer field that is divided into multiple stripes, and each stripe is divided into multiple deflection fields. Each deflection field can comprise multiple subfields. Each stripe has a length dimension and a width dimension. The width dimension corresponds to the maximal lateral deflection that can be imparted to the illumination beam by the illumination-optical system of the CPB microlithography system with which the reticle is used. Each stripe is further divided into multiple strip fields (deflection fields) that extend in the direction of the width dimension of the respective stripe.
The overall pattern-transfer field of the reticle is transferred to the substrate by deflecting and scanning the illumination beam in the direction of the width dimension of the stripes (to sequentially expose the deflection fields), while mechanically scanning (using the respective stages) the reticle and the sensitive substrate in the direction of the length dimension of the stripes (to sequentially move deflection fields into position for exposure).
A beam-drift test pattern is defined on the reticle in a deflection field at the lengthwise end of a stripe. Similarly, a corresponding beam-test mark (to be irradiated with the detection beam passing through the corresponding beam-drift test pattern on the reticle) is defined on the substrate. The apparatus includes a beam-drift-correction unit that, before pattern transfer begins, detects the positions of the beam-drift test patterns multiple times by scanning the beam-test marks on the substrate with the detection beam passing through corresponding beam-drift test patterns on the reticle. The beam-drift-correction unit iteratively determines the magnitude of beam drift, calculates the amount of required correction of the beam position, and imparts the required correction to the beam position as required during actual pattern transfer, based on the measured magnitude and direction of beam drift.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.